Heat-conductive silicone composition

ABSTRACT

A silicone composition that contains (A) an organopolysiloxane having at least two aliphatic unsaturated hydrocarbon groups per molecule, (B) a filler containing an aluminum powder and a zinc oxide powder, (C) an organohydrogenpolysiloxane having two or more silicon-bonded hydrogen atoms per molecule, and (D) a platinum group metal catalyst, in which a cured product of the silicone composition exhibits a ratio of a storage elastic modulus after 3,600 seconds from the start of measurement to a storage elastic modulus after 7,200 seconds from the start of measurement of 0.7 or less, the storage elastic modulus G′ being measured by constructing a program for holding a sample at 150° C. for 7,200 seconds after the sample is heated from 25° C. to 125° C. at a temperature increase rate of 10° C./min, from 125° C. to 145° C. at a temperature increase rate of 2° C./min, and from 145° C. to 150° C. at a temperature increase rate of 0.5° C./min.

TECHNICAL FIELD

The present invention relates to a heat-conductive silicone composition.

BACKGROUND ART

It is widely known that electronic parts such as semiconductor packagesgenerate heat in use, thereby lowering the performance thereof. To solvethis problem, various heat dissipating techniques have been used. Onetypical method is to provide a cooling member such as a heat spreader inthe vicinity of a heat-generating part and bring them into close contactto effectively remove heat through the cooling member.

In this case, if there is a space between the heat-generating member andthe cooling member, thermal conduction does not proceed smoothly becauseof the presence of air, which is poor in heat conductivity, andtherefore, the temperature of the heat-generating member cannot besufficiently reduced. To prevent such phenomena, there have beenconventionally used, for the purpose of preventing the presence of air,heat-dissipating greases or heat-dissipating sheets that have good heatconductivity and followability to the surface of the member (PatentLiteratures 1 to 11).

A thin and compressible heat-dissipating grease is suitable for measuresagainst heat of semiconductor packages in view of heat-dissipatingperformance. Particularly, it is preferable, in view of reliability, touse a thermosetting heat-dissipating grease that hardly causes outflowof the grease (pumping out) due to the thermal history between heatingand cooling of the heat-generating part. On the other hand, sinceextremely high terminal-density and a reduction in mounting area arerequired for semiconductor packages in recent years, a manner in which apackage is connected to a substrate after a reflow process for fusingsolder applied to the bottom of the package at a high temperature of200° C. or higher, is most often used instead of the conventional mannerof connecting a package and a substrate through a lead frame. However,the reflow process of subjecting to high temperature of 200° C. orhigher extremely accelerates curing of the thermosettingheat-dissipating grease. Therefore, the heat-dissipating grease cannotbe compressed into a prescribed thickness, and the appliedheat-dissipating grease cannot sufficiently spread over the wholeheat-generating part. Thus, there is a problem of insufficientheat-dissipating performance.

CITATION LIST Patent Literature

PATENT LITERATURE 1: Japanese Patent No. 2938428

PATENT LITERATURE 2: Japanese Patent No. 2938429

PATENT LITERATURE 3: Japanese Patent No. 3580366

PATENT LITERATURE 4: Japanese Patent No. 3952184

PATENT LITERATURE 5: Japanese Patent No. 4572243

PATENT LITERATURE 6: Japanese Patent No. 4656340

PATENT LITERATURE 7: Japanese Patent No. 4913874

PATENT LITERATURE 8: Japanese Patent No. 4917380

PATENT LITERATURE 9: Japanese Patent No. 4933094

PATENT LITERATURE 10: Japanese Unexamined Patent publication (Kokai) No.2012-102283

PATENT LITERATURE 11: Japanese Unexamined Patent publication (Kokai) No.2012-96361

SUMMARY OF INVENTION Technical Problem

The present invention was accomplished in view of the above-describedproblems. It is an object of the present invention to provide a siliconecomposition a cured product of which exhibits a ratio of a storageelastic modulus after 3,600 seconds from the start of measurement to astorage elastic modulus after 7,200 seconds from the start ofmeasurement of 0.7 or less when the storage elastic modulus G′ ismeasured, by means of a viscoelasticity measurement apparatus capable ofmeasuring shear modulus, by constructing a program for holding a sampleat 150° C. for 7,200 seconds after the sample is heated from 25° C. to125° C. at a temperature increase rate of 10° C./min, from 125° C. to145° C. at a temperature increase rate of 2° C./min, and from 145° C. to150° C. at a temperature increase rate of 0.5° C./min.

Solution to Problem

To achieve this object, the present invention provides a siliconecomposition comprising:

(A) 100 parts by mass of an organopolysiloxane having at least twoaliphatic unsaturated hydrocarbon groups per molecule, and having akinematic viscosity at 25° C. of 60 to 100,000 mm²/s;

(B) 100 to 2,000 parts by mass of a filler containing an aluminum powderand a zinc oxide powder;

(C) an organohydrogenpolysiloxane having two or more silicon-bondedhydrogen atoms (i.e. SiH group) per molecule, in such an amount that aratio of a number of the SiH groups in the component (C) to a totalnumber of the aliphatic unsaturated hydrocarbon groups in the component(A) ranges from 0.5 to 1.5; and

(D) a platinum group metal catalyst in an amount of 0.1 to 500 ppm interms of platinum with respect to the component (A); wherein

a cured product of the silicone composition exhibits a ratio of astorage elastic modulus after 3,600 seconds from the start ofmeasurement to a storage elastic modulus after 7,200 seconds from thestart of measurement of 0.7 or less, the storage elastic modulus G′being measured, by means of a viscoelasticity measurement apparatuscapable of measuring shear modulus, by constructing a program forholding a sample at 150° C. for 7,200 seconds after the sample is heatedfrom 25° C. to 125° C. at a temperature increase rate of 10° C./min,from 125° C. to 145° C. at a temperature increase rate of 2° C./min, andfrom 145° C. to 150° C. at a temperature increase rate of 0.5° C./min.

When such a silicone composition is employed, the heat-dissipatinggrease can be compressed into a prescribed thickness even after thereflow process of subjecting to high temperature of 200° C. or higher,and the applied heat-dissipating grease can sufficiently spread over thewhole heat-generating part. Therefore, sufficient heat-dissipatingperformance can be obtained.

In addition, the silicone composition preferably further comprises (E) 1to 200 parts by mass of a hydrolytic methylpolysiloxane represented bythe general formula (1), based on 100 parts by mass of the component(A),

wherein R⁴ represents an alkyl group having 1 to 6 carbon atoms; and “g”is an integer of 5 to 100.

When such a hydrolytic methylpolysiloxane is contained, sufficientwettability is exhibited, and curing reaction progresses sufficiently,whereby outflow of the grease can be prevented.

The silicone composition preferably further comprises (F) 0.05 to 5.0parts by mass of one or more retarders selected from the groupconsisting of acetylene compounds, nitrogen compounds, organophosphorouscompounds, oxime compounds, and organochlorine compounds, based on 100parts by mass of the component (A).

When the component (F) is contained, it functions as a retarder andsuppresses progress of the hydrosilylation reaction at room temperature,whereby shelf life or pot life can be extended.

Advantageous Effects of Invention

As described above, the silicone composition of the present inventionenables the heat-dissipating grease to be compressed into a prescribedthickness even after the reflow process of subjecting to hightemperature of 200° C. or higher, and enables the appliedheat-dissipating grease to sufficiently spread over the wholeheat-generating part. Therefore, sufficient heat-dissipating performancecan be obtained.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be described in more detail.

As described above, it has been desired to develop a siliconecomposition that enables the heat-dissipating grease to be compressedinto a prescribed thickness even after the reflow process of subjectingto high temperature of 200° C. or higher, and enables the appliedheat-dissipating grease to sufficiently spread over the wholeheat-generating part.

The present inventors diligently studied and consequently found that thesilicone composition containing the components (A) to (D) and optionallycontaining the components (E) and (F) can give a cured product thatexhibits 0.7 or less of a ratio between a storage elastic modulus after3,600 seconds from the start of measurement and a storage elasticmodulus after 7,200 seconds from the start of measurement when thestorage elastic modulus G′ is measured, by means of an viscoelasticitymeasuring apparatus capable of measuring shear modulus, by constructinga program for holding a sample at 150° C. for 7,200 seconds after thesample is heated from 25° C. to 125° C. at a temperature increase rateof 10° C./min, from 125° C. to 145° C. at a temperature increase rate of2° C./min, and from 145° C. to 155° C. at a temperature increase rate of0.5° C./min. This silicone composition enables the heat-dissipatinggrease to be compressed into a prescribed thickness even after thereflow process of subjecting to high temperature of 200° C. or higher,and enables the applied heat-dissipating grease to sufficiently spreadover the whole heat-generating part, and thus sufficientheat-dissipating performance can be obtained. From the above findings,they brought the invention to completion.

That is, the present invention is directed to a silicone compositionthat contains:

(A) 100 parts by mass of an organopolysiloxane having at least twoaliphatic unsaturated hydrocarbon groups per molecule, and having akinematic viscosity at 25° C. of 60 to 100,000 mm²/s;

(B) 100 to 2,000 parts by mass of a filler containing an aluminum powderand a zinc oxide powder;

(C) an organohydrogenpolysiloxane having two or more silicon-bondedhydrogen atoms (i.e. SiH group) per molecule, in such an amount that aratio of a number of the SiH groups in the component (C) to a totalnumber of the aliphatic unsaturated hydrocarbon groups in the component(A) ranges from 0.5 to 1.5; and

(D) a platinum group metal catalyst in an amount of 0.1 to 500 ppm interms of platinum with respect to the component (A); wherein

a cured product of the silicone composition exhibits a ratio of astorage elastic modulus after 3,600 seconds from the start ofmeasurement to a storage elastic modulus after 7,200 seconds from thestart of measurement of 0.7 or less, the storage elastic modulus G′being measured, by means of a viscoelasticity measurement apparatuscapable of measuring shear modulus, by constructing a program forholding a sample at 150° C. for 7,200 seconds after the sample is heatedfrom 25° C. to 125° C. at a temperature increase rate of 10° C./min,from 125° C. to 145° C. at a temperature increase rate of 2° C./rain,and from 145° C. to 150° C. at a temperature increase rate of 0.5°C./min.

Hereinafter, the present invention will be described in detail, but thepresent invention is not limited thereto.

Component (A)

The component (A) is an organopolysiloxane having at least two aliphaticunsaturated hydrocarbon groups per molecule, and having a kinematicviscosity at 25° C. of 60 to 100,000 mm²/s. The aliphatic unsaturatedhydrocarbon group is preferably a monovalent hydrocarbon groupcontaining aliphatic unsaturated bond and having 2 to 8 carbon atoms,more preferably 2 to 6 carbon atoms, and it is much more preferably analkenyl group. Examples thereof include alkenyl groups such as a vinylgroup, an allyl group, a propenyl group, an isopropenyl group, a butenylgroup, a hexenyl group, a cyclohexenyl group, and an octenyl group. Avinyl group is particularly preferred. The aliphatic unsaturatedhydrocarbon group may be bonded to either silicon atoms in terminals ofthe molecular chain or silicon atoms within the molecular chain, or maybe bonded to both of them.

The organopolysiloxane has a kinematic viscosity at 25° C. of 60 to100,000 mm²/s, preferably 100 to 30,000 mm²/s. If the kinematicviscosity is less than 60 mm²/s, there is fear that physical propertiesof the silicone composition are lowered, whereas if it exceeds 100,000mm²/s, the silicone composition may become poor in spreadability.

In the present invention, kinematic viscosity is a value measured byUbbelohde-type Oswald viscometer at 25° C. The molecular structure ofthe organopolysiloxane is not particularly limited so long as it has theabove-mentioned properties, and examples thereof include linearstructure, branched structure, and linear structure having partiallybranched or cyclic structure.

Particularly preferable is a linear structure in which the main chain iscomposed of diorganosiloxane repeating units and both terminals of themolecular chain are blocked with triorganosiloxy groups.Organopolysiloxane having said linear structure may have a partiallybranched or cyclic structure. The organopolysiloxane may be used solelyor in combination of two or more kinds.

An organic group bonded to silicon atoms in the organopolysiloxane otherthan the aliphatic unsaturated hydrocarbon group is a substituted orunsubstituted monovalent hydrocarbon group having 1 to 18 carbon atoms,preferably 1 to 10 carbon atoms, more preferable 1 to 8 carbon atoms.Examples thereof include alkyl groups such as methyl group, ethyl group,propyl group, isopropyl group, butyl group, isobutyl group, tert-butylgroup, pentyl group, neopentyl group, hexyl group, cyclohexyl group,octyl group, nonyl group, and decyl group; aryl groups such as phenylgroup, tolyl group, xylyl group, and naphthyl group; aralkyl groups suchas benzyl group, phenethyl group, and phenylpropyl group; and groups inwhich a part or whole of hydrogen atoms in these groups are substitutedwith halogen atoms such as fluorine, bromine, and chlorine or cyanogroups, such as chloromethyl group, chloropropyl group, bromoethylgroup, trifluoropropyl group, and cyanoethyl group. Particularly, methylgroup is preferred.

Component (B)

The component (B) is a heat-conductive filler containing an aluminumpowder and a zinc oxide powder. In the present invention, the shape ofthe aluminum powder is not particularly limited, and may be spherical,irregular shape, or other shape. The surface of the aluminum powder maybe previously treated. The aluminum powder preferably has an averageparticle size of 0.1 to 100 μm, more preferably 1 to 40 μm.

If the average particle size of the aluminum powder is 0.1 μm or more,the viscosity of the resulting composition is not too high, and thus,there is no fear that the spreadability thereof becomes poor. If it is100 μm or less, the resulting composition becomes homogeneous. In thepresent invention, as the aluminum powder, an aluminum powder having alarge average particle size or small average particle size may be usedalone, but it is preferred to mixedly used an aluminum powder having alarge average particle size (e.g. 5 μm or more and 100 μm or less,preferably 10 μm or more and 100 μm or less, more preferably 10 μm ormore and 50 μm or less) and an aluminum powder having a small averageparticle size (e.g. 0.1 μm or more and 10 μm or less, preferably 0.1 μmor more and 5 μm or less, more preferably 1 μm or more and 5 μm orless).

By mixing these, a grease with good viscosity can be obtained. Themixing ratio may be adjusted according to a desired viscosity of thegrease, and it is particularly preferred that the mass ratio of thealuminum powder having a large average particle size to the aluminumpowder having a small average particle size range from 0.5 to 9.0, morepreferably from 1.0 to 5.0.

In the present invention, the shape of the zinc oxide powder is notparticularly limited, and may be spherical, irregular shape, or othershape. The zinc oxide powder preferably has an average particle size of0.1 to 10 μm, more preferably 1 to 4 μm.

If the average particle size of the zinc oxide powder is 0.1 μm or more,there is no fear that the resulting composition exhibits a highviscosity, and that the spreadability thereof becomes poor. If it is 10μm or less, the resulting composition becomes homogeneous.

In the present invention, the “average particle size” indicates theparticle size of 50% integrated value in the particle size distributionbased on volume measured by a laser diffractive-scattering method. Themeasurement using the laser diffractive-scattering method can beperformed, for example, by a Microtrack particle size analyzer MT3300EX,(manufactured by Nikkiso Co., Ltd.).

In the present invention, (B) the filler may further contain, besidesthe aluminum powder and the zinc oxide powder, a known heat-conductivefiller such as titanium oxide powder, alumina powder, boron nitridepowder, aluminum nitride powder, diamond powder, gold powder, silverpowder, copper powder, carbon powder, nickel powder, indium powder,gallium powder, metallic silicon powder, and silica powder, depending onthe purpose.

The amount of (B) the filler is 100 to 2,000 parts by mass, preferably200 to 1,800 parts by mass, more preferably 400 to 1,800 parts by mass,based on 100 parts by mass of the component (A). If the amount of thefiller is less than 100 parts by mass, the resulting composition maybecome poor in heat conductivity. If the amount exceeds 2,000 parts bymass, the composition may become poor in spreadability.

Component (C)

The component (C) is an organohydrogenpolysiloxane having two or more,preferably three or more, particularly 3 to 100, further preferably 3 to20 silicon-bonded hydrogen atoms (i.e. SiH groups) per molecule. Theorganohydrogenpolysiloxane may be any materials so long as the SiHgroups in the molecule are subjected to addition reaction with thealiphatic unsaturated hydrocarbon groups contained in the component (A)in the presence of a later-described platinum group metal catalyst toform a crosslinking structure.

The molecular structure of the organohydrogenpolysiloxane is notparticularly limited so long as it has the above-mentioned properties,and examples thereof include linear structure, branched structure,cyclic structure, and linear structure having partially branched orcyclic structure. Particularly preferable is linear or cyclic structure.The organohydrogenpolysiloxane preferably has a kinematic viscosity at25° C. of 1.0 to 1,000 mm²/s, more preferably 10 to 100 mm²/s.

If the kinematic viscosity is 1.0 mm²/s or more, there is no fear thatphysical properties of the silicone composition are lowered. If it is1,000 mm²/s or less, there is no fear that the spreadability of thesilicone composition becomes poor. In the present invention, thekinematic viscosity is a value measured by Ubbelohde-type Oswaldviscometer at 25° C.

As the organic group bonded to silicon atoms in theorganohydrogenpolysiloxane, there may be mentioned a monovalenthydrocarbon group other than an aliphatic unsaturated hydrocarbon group,in particular, an unsubstituted or substituted monovalent hydrocarbongroup having 1 to 12 carbon atoms, preferably 1 to 10 carbon atoms.Examples thereof include alkyl groups such as methyl group, ethyl group,propyl group, butyl group, hexyl group, and dodecyl group; aryl groupssuch as phenyl group; aralkyl groups such as 2-phenethyl group and2-phenylpropyl group; groups in which a part or whole of hydrogen atomsin these groups are substituted with halogen atoms such as fluorine,bromine, and chlorine or cyano groups, such as chloromethyl group,chloropropyl group, bromoethyl group, trifluoropropyl group, andcyanoethyl group; and epoxy ring-containing organic groups (alkyl groupssubstituted with glycidyl group or glycidyloxy group) such as2-glycidoxyethyl group, 3-glycidoxypropyl group, and 4-glycidoxybutylgroup. The organohydrogenpolysiloxane may be used solely or incombination of two or more kinds.

The amount of (C) the organohydrogenpolysiloxane is such an amount thatthe ratio of a number of the SiH groups in the component (C) to a totalnumber of the aliphatic unsaturated hydrocarbon groups in the component(A) is in the range of 0.5 to 1.5. If the amount of the component (C) isless than 0.5, there is fear that the curing reaction does not proceedsufficiently, and thus outflow of the grease occurs. If it exceeds 1.5,unreacted SiH groups cause excess crosslinking reaction, so that itbecomes difficult to compress the heat-dissipating grease into aprescribed thickness after the reflow process and to sufficiently spreadthe applied heat-dissipating grease over the whole heat-generating part,which may result in insufficient heat-dissipating performance.

Component (D)

The component (D) is a platinum group metal catalyst, which serves toaccelerate the addition reaction mentioned above. As the platinum groupmetal catalyst, conventionally known materials used for additionreaction can be used. Examples thereof include platinum-based,palladium-based, and rhodium-based catalyst. Among them, platinum and aplatinum compound are preferred because of relatively high availabilitythereof. For example, simple platinum, platinum black, chloroplatinicacid, platinum-olefin complexes, platinum-alcohol complexes, andplatinum coordination compounds may be mentioned. The platinum-basedcatalyst may be used solely or in combination of two or more kinds.

The amount of the component (D) is 0.1 to 500 ppm, preferably 1.0 to 100ppm in terms of platinum group metal atoms relative to the component (A)on the basis of mass. If the amount of the catalyst is less than 0.1ppm, no catalytic effect is expected. If it exceeds 500 ppm, thecatalytic effect does not increase, resulting in poor economy, so thatit is not preferable.

Component (E)

The silicone composition of the present invention may further contain ahydrolytic methylpolysiloxane represented by the formula (1).

In the formula (1), R⁴ represents an alkyl group having 1 to 6 carbonatoms; and examples thereof include methyl group, ethyl group, propylgroup, isopropyl group, butyl group, sec-butyl group, tert-butyl group,pentyl group, and hexyl group. “g” is an integer of 5 to 100, preferably10 to 60. If the value of “g” is 5 or more, there is no fear thatoil-bleeding from the silicone composition becomes prominent andreliability is lowered, so that it is preferable. If the value of “g” is100 or less, there is no fear that the wettability to the filler becomesinsufficient, so that it is preferable.

The amount of the component (E) is 1 to 200 parts by mass, preferably 10to 180 parts by mass, more preferably 20 to 150 parts by mass, based on100 parts by mass of the component (A). If the amount of the component(E) is 1 part by mass or more, sufficient wettability can be expressed.If the amount of the component (E) is 200 parts by mass or less, curingreaction proceeds sufficiently, so that outflow of the grease does notoccur. Therefore, it is preferable.

Component (F)

The silicone composition of the present invention may further containcomponent (F), a retarder. The retarder serves to suppress the progressof the hydrosilylation reaction at room temperature, and is used forextending shelf life or pot life. As the retarder, a conventionallyknown retarder used for an addition-curable silicone composition can beused. Examples thereof include acetylene compounds such as acetylenealcohols (e.g., 1-ethynyl-1-cyclohexanol and 3,5-dimethyl-1-hexyn-3-ol);various nitrogen compounds such as tributylamine,tetramethylethylenediamine, and benzotriazol; organophosphorus compoundssuch as triphenylphosphine; oxime compounds; and organochlorinecompounds.

The amount of the component (F) is 0.05 to 5.0 parts by mass, preferably0.1 to 1.0 part by mass, based on 100 parts by mass of the component(A). If the amount of the retarder is 0.05 part by mass or more, adesired sufficient shelf life or pot life can be obtained. If it is 5.0parts by mass or less, there is no fear that curability of the siliconecomposition is lowered, so that it is preferable.

The retarder may be diluted with toluene or the like to enhancedispersibility to the silicone composition. Moreover, the siliconecomposition of the present invention may contain unreactiveorgano(poly)siloxane such as methylpolysiloxane to adjust elasticity andviscosity of the composition.

In addition, to prevent deterioration of the silicone composition, aconventionally known antioxidant such as 2,6-di-t-butyl-4-methylphenolmay be added, if necessary. Further, a dye, a pigment, a flameretardant, a precipitation-inhibitor, a thixotropy-enhancer, or otheradditives may be blended, if necessary.

A method for producing the silicone composition of the present inventionmay follow the conventional method for producing a silicone greasecomposition, and is not particularly limited. For example, thecomposition can be produced by mixing the components (A) to (F) and anyother optional components with a mixer such as a Trimix, Twinmix orPlanteary Mixer (all registered trademarks for mixers manufactured byInoue Manufacturing Co., Ltd.), an Ultramixer (a registered trademarkfor a mixer manufactured by Mizuho Industrial Co., Ltd.), and a HivisDisper Mix (a registered trademark for a mixer manufactured by PRIMIXCorporation).

The silicone composition of the present invention preferably has aviscosity measured at 25° C. of 3.0 to 300 Pa·s, more preferably 5.0 to200 Pa·s. The viscosity of 3.0 Pas·s or more is preferable because thereis no fear that the workability is lowered. The above viscosity can beachieved by adjusting the composition of the respective components. Inthe present invention, the viscosity is a value measured at 25° C. by aMalcolm viscometer (at 10 rpm with Rotor-A, and at a shear rate of6[1/s]).

The silicone composition of the present invention can be suitably used,as well as the conventional heat-conductive silicone grease, fordissipating heat by placing the composition between a cooling member anda heat-generating member such as electronic parts of a semiconductorpackage to conduct heat from the heat-generating member to the coolingmember. In particular, it is effective to the mounting form requiringthe reflow process for fusing solder applied to the bottom of thepackage at a high temperature of 200° C. or higher. The curingconditions when the silicone composition of the present invention isthermally cured are not particularly limited, but generally 80° C. to200° C., preferably 100 to 180° C., for 30 minutes to 4 hours,preferably 30 minutes to 2 hours.

EXAMPLES

Hereinafter, the present invention will be more specifically describedwith reference to examples and comparative examples, but the presentinvention is not limited to the following examples. Tests relating tothe effects of the present invention were performed in the mannerdescribed below.

[Storage Elastic Modulus]

The silicone composition was applied between two parallel plates eachhaving a diameter of 2.5 cm so as to give a thickness of 2 mm. Then, aprogram was constructed for heating the coated plates from 25° C. to125° C. at a temperature increase rate of 10° C./min, from 125° C. to145° C. at a temperature increase rate of 2° C./min, and from 145° C. to150° C. at a temperature increase rate of 0.5° C./min and then holdingthem at 150° C. for 7,200 seconds; and storage elastic modulus after3,600 seconds and 7,200 seconds from the start of holding at 150° C.were measured. The measurement was performed using a viscoelasticitymeasurement apparatus (Type: RDAIII, manufactured by RheometricScientific Ltd.).

[Viscosity]

The absolute viscosity of the silicone composition was measured by aMalcolm viscometer (type: PC-1T) at 25° C.

[Heat Conductivity]

The silicone composition was wrapped with kitchen wrap and measured byTPA-501 manufactured by Kyoto electronics manufacturing Co., Ltd.

[Crushability Test]

5 mg of the silicone composition was applied onto a silicon wafer (10mm×10 mm), and heated at 240° C. for 2 minutes. Thereafter, the waferwas cooled to room temperature, and a silicon wafer with the same sizewas placed thereon to prepare a test sample pressurized at 1.8 kgf for 5seconds. Then, the thickness of the silicone composition was measured.

[Spreadability Test]

The test sample used in the crushability test was separated into twosilicon wafers, and (area covered with the silicone composition)/(areaof the silicon wafer) was calculated.

The respective components used in examples and comparative examples areshown below. In the following, the kinematic viscosity is a valuemeasured by Ubbelohde-type Oswald viscometer (manufactured by Sibatascientific technology Ltd.) at 25° C.

Component (A)

A-1: dimethylpolysiloxane having a kinematic viscosity at 25° C. of 600mm²/s in which both terminals are blocked with dimethylvinylsilylgroups.

A-2: dimethylpolysiloxane having a kinematic viscosity at 25° C. of 600mm²/s in which both terminals are blocked with trimethylsilyl groups andtwo of methyl groups bonded to silicon atoms within the molecular chainare vinyl groups.

Component (B)

B-1: an aluminum powder having an average particle size of 20.0 μm (heatconductivity: 237 W/m·° C.)

B-1: an aluminum powder having an average particle size of 2.0 μm (heatconductivity: 237 W/m·° C.)

B-3: a zinc oxide powder having an average particle size of 1.0 μm (heatconductivity: 25 W/m·° C.)

Component (C)

Component (D)

D-1: a solution in which a platinum-divinyltetramethyldisiloxane complexwas dissolved in the same dimethylpolysiloxane as A-1 (platinum atomcontent: 1% by mass).

Component (E)

Component (F)

Examples 1 to 13 and Comparative examples 1 to 7

The components (A) to (F) were mixed as described below to obtaincompositions of Examples 1 to 13 and Comparative examples 1 to 7.

That is, the components (A), (B), and (E) were placed into a 5-LPlanteary Mixer (manufactured by Inoue Manufacturing Co., Ltd.) with theblending ratio shown in Tables 1 and 2, and mixed for 1 hour at roomtemperature. Then, the components (C), (D), and (F) were added thereto,and mixed homogeneously. The measurement results of storage elasticmodulus, viscosity, and heat conductivity and results of thecrushability test and the spreadability test of the obtained compositionwere shown in Tables 1 and 2.

Examples 1 to 13

TABLE 1 1 2 3 4 5 6 7 Composition A-1 100 100 100 100 100 100 (parts bymass) A-2 100 B-1 490 490 654 368 490 490 490 B-2 324 324 432 244 324324 324 B-3 100 100 134 76 100 100 100 Total amount of 914 914 1220 688914 914 914 filler C-1 7.9 7.7 7.9 7.9 3.2 9.5 11.9 C-2 6.9 D-1 0.060.06 0.06 0.06 0.06 0.06 0.06 E-1 100 100 100 100 100 100 100 F-1 0.40.4 0.4 0.4 0.4 0.4 0.4 SiH/SiVi 1.0 1.0 1.0 1.0 1.0 1.2 1.5 (numberratio) Storage elastic 14650 9310 19690 10920 14950 53060 180080 modulus(after 3,600 sec) Storage elastic 55560 46320 70440 32500 28810 183110486640 modulus (after 7,200 sec) after 3,600 sec/ 0.26 0.20 0.28 0.340.52 0.29 0.37 after 7,200 sec Viscosity 9.5 9.1 24 5.2 9.3 9.1 8.8 (Pa· S) Heat 2.1 2.0 2.7 1.2 2.1 2.0 1.9 conductivity (W/m · ° C.)Evaluation Thickness of 34 36 48 34 46 39 43 result silicone compositionafter crushability test (μm) Result of 0.88 0.87 0.66 0.91 0.71 0.800.72 spreadability test (area covered with silicone composition/area ofsilicon wafer) 8 9 10 11 12 13 Composition A-1 100 100 100 100 100 100(parts by mass) A-2 B-1 490 490 490 490 490 490 B-2 324 324 324 324 324324 B-3 100 100 100 100 100 100 Total amount of 914 914 914 914 914 914filler C-1 7.9 7.9 7.9 7.9 5.5 4.0 C-2 D-1 0.09 0.06 0.06 0.06 0.06 0.06E-1 100 100 150 50 100 100 F-1 0.4 0.6 0.4 0.4 0.4 0.4 SiH/SiVi 1.0 1.01.0 1.0 0.7 0.5 (number ratio) Storage elastic 60780 3360 2110 1257007390 2950 modulus (after 3,600 sec) Storage elastic 93320 7700 13940220480 13640 4480 modulus (after 7,200 sec) after 3,600 sec/ 0.65 0.440.15 0.57 0.54 0.66 after 7,200 sec Viscosity 9.4 9.6 5.8 45 9.7 9.8 (Pa· S) Heat 2.0 2.0 1.6 2.6 2.0 2.1 conductivity (W/m · ° C.) EvaluationThickness of 47 33 30 44 35 34 result silicone composition aftercrushability test (μm) Result of 0.67 0.94 0.97 0.69 0.86 0.91spreadability test (area covered with silicone composition/area ofsilicon wafer)

Comparative Examples 1 to 7

TABLE 2 1 2 3 4 5 6 7 Composition A-1 100 100 100 100 100 100 100 (partsby mass) A-2 B-1 1180 490 490 490 490 490 490 B-2 778 324 324 324 324324 324 B-3 240 100 100 100 100 100 100 Total amount of 2198 914 914 914914 914 914 filler C-1 7.9 3.2 59 7.9 16 20 C-2 4.6 D-1 0.06 0.06 0.060.06 10 0.06 0.06 E-1 100 100 100 100 100 100 100 F-1 0.4 0.4 0.4 0.40.4 0.4 0.4 SiH/SiVi 1.0 0.4 0.4 7.5 1.0 2.0 2.5 (number ratio) Storageelastic Not Not Not 521000 75960 249530 388160 modulus becoming curedcured (after 3,600 sec) grease Storage elastic form 659240 88030 318770482700 modulus (after 7,200 sec) after 3,600 sec/ 0.79 0.86 0.78 0.80after 7,200 sec Viscosity 6.5 9.7 8.4 8.2 (Pa · S) Heat 1.5 2.1 1.9 1.8conductivity (W/m · ° C.) Evaluation Thickness of 87 93 77 81 resultsilicone composition after crushability test (μm) Result of 0.36 0.350.42 0.37 spreadability test (area covered with siliconecomposition/area of silicon wafer)

From the results of Tables 1 and 2, Examples 1 to 13 satisfying therequirements of the present invention show smaller thicknesses of thesilicone composition after the crushability test and larger resultingvalues of the spreadability test, compared with Comparative examples 1to 7. Accordingly, it could be confirmed that the silicone compositionof the present invention enables the heat-dissipating grease to becompressed into a prescribed thickness even after the reflow process ofsubjecting to high temperature of 200° C. or higher, and enables theapplied heat-dissipating grease to sufficiently spread over the wholeheat-generating part.

It is to be noted that the present invention is not limited to theforegoing embodiment. The embodiment is just an exemplification, and anyexamples that have substantially the same feature and demonstrate thesame functions and effects as those in the technical concept describedin claims of the present invention are included in the technical scopeof the present invention.

1-3. (canceled)
 4. A silicone composition comprising: (A) 100 parts bymass of an organopolysiloxane having at least two aliphatic unsaturatedhydrocarbon groups per molecule, and having a kinematic viscosity at 25°C. of 60 to 100,000 mm²/s; (B) 100 to 2,000 parts by mass of a fillercontaining an aluminum powder and a zinc oxide powder; (C) anorganohydrogenpolysiloxane having two or more silicon-bonded hydrogenatoms (i.e. SiH group) per molecule, in such an amount that a ratio of anumber of the SiH groups in the component (C) to a total number of thealiphatic unsaturated hydrocarbon groups in the component (A) rangesfrom 0.5 to 1.5; and (D) a platinum group metal catalyst in an amount of0.1 to 500 ppm in terms of platinum with respect to the component (A);wherein a cured product of the silicone composition exhibits a ratio ofa storage elastic modulus after 3,600 seconds from the start ofmeasurement to a storage elastic modulus after 7,200 seconds from thestart of measurement of 0.7 or less, the storage elastic modulus G′being measured, by means of a viscoelasticity measurement apparatuscapable of measuring shear modulus, by constructing a program forholding a sample at 150° C. for 7,200 seconds after the sample is heatedfrom 25° C. to 125° C. at a temperature increase rate of 10° C./min,from 125° C. to 145° C. at a temperature increase rate of 2° C./min, andfrom 145° C. to 150° C. at a temperature increase rate of 0.5° C./min.5. The silicone composition according to claim 4, further comprising (E)1 to 200 parts by mass of a hydrolytic methylpolysiloxane represented bythe general formula (1), based on 100 parts by mass of the component(A),

wherein R⁴ represents an alkyl group having 1 to 6 carbon atoms; and “g”is an integer of 5 to
 100. 6. The silicone composition according toclaim 4, further comprising (F) 0.05 to 5.0 parts by mass of one or moreretarders selected from the group consisting of acetylene compounds,nitrogen compounds, organophosphorous compounds, oxime compounds, andorganochlorine compounds, based on 100 parts by mass of the component(A).
 7. The silicone composition according to claim 5, furthercomprising (F) 0.05 to 5.0 parts by mass of one or more retardersselected from the group consisting of acetylene compounds, nitrogencompounds, organophosphorous compounds, oxime compounds, andorganochlorine compounds, based on 100 parts by mass of the component(A).